Microstrip cross-finger filters, as a common two-dimensional filter, have a wide range of engineering applications, but due to their large size, thus limiting the expansion of application scenarios. In this paper, a new microstrip cross-finger filter is designed based on the theory of step impedance resonator (SIR), which completes the optimization of the size of the conventional microstrip cross-finger filter. The filter uses a three-section SIR structure, which reduces the length by 23.2% compared to the resonator of a conventionally structured filter. The performance of SIR filter and traditional filter under the same design index is simulated using CST, and the results show that both of them can meet the design index requirements, and the performance of SIR filter is better in the passband, which indicates that the use of this method to realize the miniaturization has strong practical significance. In order to verify the usability of the filter, physical processing and testing were carried out with a board size of 24mm×18mm×0.508mm, and the difference between the measured and simulated results is less than 2dB at the passband. The final specifications of this filter are: center frequency 2.5GHz, bandwidth 150MHz, in-band loss ≤4dB, and stopband rejection ≥24dB (frequency ≤2.2GHz, ≥2.72GHz).